The present invention is directed to frequency hopping wireless communications systems. More specifically, the present invention reduces interference levels and increases capacity in frequency hopping wireless communications systems by dynamically replacing system frequencies in use within selected frequency hop patterns with system frequencies having lower interference levels and by precluding nearby interfering system components (for example, base stations) from simultaneously making frequency replacements using the same available system frequencies.
The demand for wireless communications services continues to grow at an astonishing rate. For example, each day a greater percentage of the public elects untethered access to a telephone system using cellular telephones. Unlike traditional telephones with attached cords which limit the user""s movement, cellular telephones allow users to make telephone calls while in transit between locations. In addition to wireless voice communication services, the public is discovering numerous instances where wireless data communication simplifies their lives. For example, an employee who has traveled on business away from a local area network (LAN) in the home office may use a laptop computer having a radio transceiver to establish a wireless connection to the LAN from within a hotel room. Once the wireless connection is established, the employee may check electronic mail or access other files on the LAN in the same manner these tasks might be performed from within the home office using a desktop computer wired to the LAN.
Unfortunately, the number of frequencies available to support the public""s growing appetite for wireless communications services is limited. Thus, service providers must make the most efficient use of these frequencies to meet the growing demand. One method for increasing the efficiency of a wireless communication system entails avoiding the use of frequencies with high interference levels which might otherwise require that data be re-transmitted thereby consuming additional system resources. Some of the current wireless communication systems implement some type of frequency hopping technique to reduce the system-wide impact of frequencies which are experiencing high interference levels. As explained below, however, the current methods for implementing frequency hopping techniques leave room for improvement.
Understanding the current methods for implementing frequency hopping techniques requires a basic understanding of how typical wireless communication systems transmit data and the major sources of interference within these systems. In a typical wireless communication system, a transmitter modulates a carrier frequency with voice or data information and transmits the modulated carrier frequency through the air to a receiver. The receiver then demodulates the carrier frequency to obtain the included voice or data information. In some wireless systems, the receiver sends the transmitter a message which indicates whether the transmitted data was successfully received. Co-channel interference, multipath fading, and shadow fading are among the types of interference which may prevent the receiver from successfully receiving transmitted data.
Co-channel interference may result when two transmitters within range of each other attempt to transmit data to their respective receivers using the same carrier frequency at the same time. The greater the level of co-channel interference, the greater the chance the transmitted data will become too distorted for the receiver to process. System resources required to retransmit this data are unavailable to transmit newly arriving data. As a result, the flow of data through the system is slowed. As the number of users in a wireless system using the limited available number of frequencies continues to increase, the possibility that two or more transmitters may be located within range of each other and transmit data using the same frequency at the same time also increases. Co-channel interference is particularly relevant to the design and deployment of cellular wireless systems.
Multipath fading occurs when a transmitted signal is reflected by objects in the path between the transmitter and receiver. As a result of one or more reflections, multiple versions of the transmitted signal may arrive at the receiver at different times. The division of the transmitted signal into these multiple versions may cause the amplitude of the transmitted signal to fade at the receiver. If the level of fading is great enough, the strength of the signal arriving at the receiver may be too low for proper receiver processing and the signal may need to be retransmitted.
Shadow fading is caused by vehicles moving in and out from behind buildings, hills, and other obstructions. Shadow fading changes at a rate of about once per second.
Both co-channel interference and multipath fading are frequency dependent. For example, two in-range transmitters may transmit at the same time without interfering with each other if each transmitter uses a different frequency. With respect to multipath fading, some ranges of frequencies are more susceptible to fading than others when transmitted along the same path. Thus, some wireless communications systems constantly xe2x80x9chopxe2x80x9d from one available carrier frequency to another available carrier frequency while transmitting data to avoid the prolonged use of a frequency which might be experiencing high interference levels. Current frequency hopping systems select frequencies at the time a call is initiated. Prior to hopping from one frequency to another, the transmitting device will usually send a message to the receiving device so the receiving device will anticipate receiving data on the new frequency. Depending on the wireless system, the pattern the transmitter follows while hopping among available frequencies may be preplanned, random, pseudorandom, or based upon dynamic frequency interference level measurements. Further, when a receiver switches from communicating with one transmitter to communicating with another transmitter, the frequency hop patterns will likely change.
Some frequency hopping wireless systems continually measure interference levels for selected system frequencies during system operation. These xe2x80x9cdynamicxe2x80x9d interference level measurements may be used to substitute frequencies experiencing high interference levels with frequencies having lower interference levels. For example, U.S. Pat. No. 5,323,447 to Mark E. Gillis et al describes a frequency hopping method in which a cordless telephone handset measures interference levels among a first group of frequencies within a frequency hop pattern while using the first group of frequencies to communicate with a base unit. When interference is detected on one of the frequencies in the first group, the base station replaces that frequency with a frequency (from a second group of frequencies) having a lower interference level. In another example, U.S. Pat. No. 5,394,433 to David F. Bantz et al, discloses a frequency hopping method in which the entire frequency hop pattern currently in use by a base station and a mobile station is replaced with a new frequency hop pattern from a predetermined set of patterns upon detecting frequencies with an unacceptable interference level within the current frequency hop pattern.
Unfortunately, current dynamic frequency hop management methods measure each system frequency sequentially. Due to the rate at which interference levels may be sequentially measured for each system frequency, these current frequency hop management methods also do not contemplate measuring all system frequencies at a rate near the rate at which the power of a received frequency signal fades while propagating through the transmission medium or the rate at which co-channel interference changes. The medium through which a frequency signal is transmitted influences the strength of the signal at the receiver. The Rayleigh fading rate is typically used to describe the statistical time varying nature of frequency signals transmitted through the air. Although the Rayleigh fading rate covers a range of rates, a fading rate of 100 Hz (which translates to a period of approximately 10 ms) is typically used to describe the rate at which the power of a received frequency signal varies while propagating through the air. Current frequency hop management methods are only able to measure interference levels for a small portion of the total number of frequencies available to a typical wireless system within a period during which channel and interference changes occur. Thus, current frequency management methods make replacement decisions by selecting high quality frequencies from among fewer than the total number of frequencies available to the system and make replacement decisions with insufficient knowledge of both the propagation medium and interference behavior. The ability to measure interference levels for all available system frequencies at a rate faster than once per second enables a frequency management method to take full advantage of the potential benefits of frequency hopping techniques by selecting best quality frequencies from among all system frequencies when attempting to reduce the impact of both propagation medium and interference influences on the quality of system frequencies.
Additionally, current frequency hop management methods do not eliminate the possibility that two interfering transmitters within the same system may respond to measured interference levels by simultaneously switching to the same high quality frequencies and again interfering with each other""s transmissions.
In view of the above, it can be appreciated that there is a need for a method and apparatus that solves the above mentioned problems.
The present invention provides a method and apparatus for reducing interference in a frequency hopping wireless communication system. According to an embodiment of the present invention, a wideband transceiver and an orthogonal frequency division multiplexing (OFDM) technique are used to simultaneously measure an interference level for each system frequency. After simultaneously measuring interference levels for each system frequency using a base station and a terminal station communicating with the base station, the base station identifies a frequency hop pattern currently in use for each base station/terminal station communication link. The measured frequency interference levels are then used to identify each frequency hop pattern in which at least one of the current system frequencies should be replaced with a system frequency having a lower interference level. The base station then replaces no more than a predetermined number of the current system frequencies within the identified frequency hop pattern(s) with system frequencies having lower interference levels. The above steps are executed independently for uplink and downlink frequency hop patterns at each base station within the wireless system while ensuring that nearby mutually interfering base stations do not replace frequency hop pattern frequencies at the same time.